The substance that combine chemically in a fixed ratio are called__________.

Options

A. Atoms
B. Elements
C. Molecules
D. Compounds

Answer

To answer the question "The substance that combine chemically in a fixed ratio are called__________," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Question**: The question is asking for a term that describes substances that combine chemically in a fixed ratio.

2. **Identifying Key Terms**: The key terms here are "combine chemically" and "fixed ratio." This indicates that we are looking for a specific type of chemical combination.

3. **Reviewing Options**: The possible options mentioned in the video are:
- Atoms
- Elements
- Molecules
- Compounds

4. **Analyzing Each Option**:
- **Atoms**: Atoms are the basic units of matter and cannot combine in a fixed ratio by themselves; they form elements or compounds.
- **Elements**: Elements consist of one type of atom and do not combine in a fixed ratio with themselves; they are pure substances.
- **Molecules**: Molecules are formed when two or more atoms bond together, but they do not necessarily combine in a fixed ratio.
- **Compounds**: Compounds are substances formed when two or more different elements combine chemically in a fixed ratio.

5. **Applying Dalton's Theory**: According to Dalton's atomic theory, compounds are formed when elements combine in specific ratios. This supports the idea that the correct answer relates to compounds.

6. **Conclusion**: Based on the analysis, the substances that combine chemically in a fixed ratio are called **compounds**.

### Final Answer:
The substance that combine chemically in a fixed ratio are called **compounds**.

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